[POWERPC] Advertise correct IDE mode on Pegasos2

The built-in IDE controller is configured in legacy mode, but the PCI
registers advertise native mode.  Force the PCI class into legacy
mode. This allows pata_via to access two drives.

The Pegasos specific irq enforcement in the via82cxxx driver must stay
because there is apparently no generic way to setup irq per channel.

Tested on Pegasos2 with firmware version 20040810, and two IDE disks.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>